There is a news article recently posted about window films in the news/blogs section.  What are peoples' experiences with them?  Can they be a cheaper way to jazz up some so-so windows and get you much of the benefit of much more expensive windows?





Looking Through (Insulated) Glass for Energy Savings
Posted By: Jamie  on 07/21/2009

Window film is cool stuff. Literally. It reflects the sun and insulates buildings. It is used to keep buildings cool on hot, sunny days, and to keep heat from escaping in cold climates.

July 20, 2009, by Katie Schindall, MBA/MEM candidate, Yale University, 2009 Climate Corps fellow at Sodexo, member of Net Impact.
Originally published on the EDF Innovation Exchange Blog

According to an article published on FacilitiesNet in 2006, window film can reduce heat penetration up to 80 percent and improve insulation by as much as 32 percent for single-pane windows or as much as 23 percent for double-pane glass. Anecdotal evidence repeatedly notes increased employee comfort from the decreased solar glare and better insulation.

If they are installed correctly, window films can protect against UV light and help insulate (minimally).  However, installing them on the outside of the window panel isn't as effective as getting new windows.  Two reasons:

1)  Most heat transfer occurs in the window frame.  When R-value is measured, it is taken from the middle of the window, farthest away from the window frame where heat transference can occur. 

2)  Some windows, like Serious Windows, have the window film located between the two glass panes.  This adds an extra layer to reduce heat and air transference, and the film is 99% effective against UV light (vs. 80% or 90% effective from other outside films). 

im not too sure about window film on insulated glass becouse the film is installed on the inside of the window the sun can build up too much heat on the thermal pane somtimes resulting in a heat crack in the glass or failing of the thermal seal

The big window film manufactures warrant against breakage, since it can happen because heat does build up. But what happens to the seals from the extra heat?

We have window film installed and deeply regret it. We thought it would help with energy and privacy issues. Now I think the opposite is the case because it's so dark in the house now that we pretty much keep the lights on all the time maybe spending more for lighting then the savings account for. Would never do it again.

The VERY highly reflective mirror like privacy or highest 70%+ heat rejection versions cut out quite a bit more light than the lighter-duty ~50% versions- I'm not sure I'd ever go there either.  But unless you're using low-efficiency lighting or have a sub-500 cooling-degree-day climate odds are pretty good it's still saving a significant amount of cooling power (and some heating well.) 

A south-pitch 10 square foot skylight is like a 1000W space heater for 4 hours out of a mid-summer day, which takes 250-300W of cooling power to keep up with.  With a 75% heat rejection film you cut that cooling power to under 100 watts.  Are you really making up for the daylight loss with a lamp loads of over 200 watts?

If yes, you probably would have done better with a ~50% heat rejection film cutting the the peak cooling power for that skylight to under 150watts and kept the lights off.  Alternatively, going with high-efficiency linear fluorescents with dimmable ballasts is roughly half the power of compact fluorescents. There are even "daylight harvesting" versions to maintain the room ambient within a reasonable range, etc.

May I ask which manufacturer/model film you went with?